Study of GaN Based Nanostructures and Hybrids
Sammanfattning: GaN and its alloys with Al and In belong to the group III nitride semiconductors and are today the materials of choice for efficient white light emitting diodes (LEDs) enabling energy saving solid state lighting. Currently, there is a great interest in the development of novel inexpensive techniques to fabricate hybrid LEDs combining high quality III-N quantum well (QW) structures with inexpensive colloidal nanoparticles or conjugated polymers. Such hybrid devices are promising for future micro-light sources in full-color displays, sensors and imaging systems. Organics can be engineered to emit at different wavelengths or even white light based on functional groups or by blend of several polymers. This is especially important for the green region, where there is still a lack of efficient LEDs. Besides optoelectronics, other applications such as biochemical sensors or systems for water splitting can be realized using GaN-based nanostructures. Despite a significant progress in the field, there is still a need in fundamental understanding of many problems and phenomena in III-nitride based nanostructures and hybrids to fully utilize material properties on demand of specific applications.In this thesis, hybrid structures based on AlGaN/GaN QWs and colloidal ZnO nano-crystals have been fabricated for down conversion of the QW emission utilizing non-radiative (Förster) resonant energy transfer. Time-resolved photoluminescence (TRPL) was used to investigate the QW exciton dynamics depending on the cap layer thickness in the bare QW and in the hybrid samples. Although the surface potential influences the exciton dynamics, the maximum pumping efficiency assuming a non-radiative energy transfer mechanism was estimated to be ~40 % at 60 K in the structure with thin cap layer of 3 nm.Since bulk GaN of large area is difficult to synthesize, there is a lack of native substrates. Thus, GaN-based structures are usually grown on SiC or sapphire, which results in high threading dislocation density in the active layer of the device and can be the reason of efficiency droop in GaN based LED structures. Fabricating GaN nanorods (NR) can be a way to produce GaN with lower defect density since threading dislocations can be annihilated toward the NR wall during growth. Here, GaN(0001) NRs grown on Si(111) substrates by magnetron sputtering using a liquid Ga target have been investigated. A high quality of NRs have been confirmed by transmission electron microscopy (TEM) and TRPL. Two strong near band gap emission lines at ~3.42 eV and ~3.47 eV related to basal plane stacking faults (SF) and donor-bound exciton (DBE), respectively, have been observed at low temperatures. TRPL properties of the SF PL line suggest that SFs form a regular structure similar to a multiple QWs, which was confirmed by TEM. The SF related PL measured at 5 K for a single NR has a significantly different polarization response compared to the GaN exciton line and is much stronger polarized (> 40 %) in the direction perpendicular to the NR growth axis.Hybrids fabricated using GaN NRs and the green emitting polyfluorene (F8BT) have been studied using micro-TRPL. In contrast to the DBE emission, the recombination time of the SF-related emission was observed to decrease, which might be due to the Förster resonance energy transfer mechanism.Compared to chemical vapor deposition, sputtering allows synthesis at much lower temperatures. Here, sputtering was employed to grow InAlN/GaN heterostructures with an indium content targeted to ~18 %, which is lattice matched to GaN. This means that near strain-free GaN films can be synthesized. It was found that using a lower temperature (~25 C) while depositing the top InAlN results in an improved interface quality compared to deposition at 700 C. In latter case, regions of quaternary alloy of InAlGaN forming structural micro-defects have been observed at the top InAlN/GaN interface in addition to optically active flower-like defect formations.
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